Positive-displacement rotary pump

ABSTRACT

A positive-displacement rotary pump comprising a pair of meshing gears ( 13, 14 ) or rotors, consisting of a driving gear and a driven gear, contained in a shell ( 10 ) having an output opening and an intake opening for a fluid. The gears ( 13, 14 ) comprise shafts ( 23, 24 ) which are supported by bushings ( 15   a   , 15   b ) having two faces ( 17   a   , 17   b ) which are subjected, in use, to pressures which bring about an axial load on the bushing itself, characterized in that the resultant of the axial loads (S′, S″) on the two bushings has a predetermined direction so as to move the bushings ( 15   a,    15   b ) and the gears ( 13, 14 ) as a whole into close abutment with a predetermined reference plane (VI-VI).

[0001] The present invention relates to the sector ofpositive-displacement rotary pumps. There are known various types ofrotary pump, including gear pumps, lobe pumps and screw pumps.

[0002] Gear pumps generally comprise two gears, one of which, known asthe driving gear, is connected to a drive shaft and causes the otherwheel, known as the driven gear, to rotate. The pumps of this type forhigh pressures are generally produced with a so-called “balanced” or“equilibrated” configuration, in which the two opposing faces of thebushings for supporting-the gears are subjected to pressures over areaswhich, although they are large in absolute terms, are not very differentfrom each other in order to generate a moderate differential force whichtends to keep each bushing in contact with the gears.

[0003]FIGS. 1 and 2 illustrate an example of a gear pump of known type.In particular, FIG. 1 is a longitudinal section along a plane whichextends through the axes of rotation of the two gears, and FIG. 2 is asection taken on line II-II in FIG. 1. The driving gear 13 and thedriven gear 14, whose shafts are supported by two bushings 15, arehoused inside a shell 10 which is closed by a front cover 11 and a rearcover 12. Omega-shaped ({overscore (ω)}) seals 16 which separate theintake zone (A), at lower pressure, from the output zone (M), at higherpressure, are housed on the outer face 17 a of the bushings 15. Duringuse, the bushings 15 are subjected to a pressure both on the outer faces17 a thereof and on the inner faces 17 b thereof. The omega-likeconfiguration of the seals 16 is such that the portion of outer face 17a of each bushing 15 on which the output pressure, which is greater thanthe intake pressure, acts is greater than the portion of inner face 17 bof the bushing which is subjected to the same output pressure. Since thearea on which the output pressure acts in the region of the inner face17 b of each bushing cannot be determined with accuracy, the optimumconfiguration of the omega-like seal 16 is usually identified by trialand error.

[0004] Owing to the difference between the pressures which act on thetwo faces, the outer face 17 a and inner face 17 b, the bushings 15 areurged with a force which is moderate, and controlled, against the gears13 and 14 so as to minimize the leakages over the faces of the gearsthemselves as a result of the difference in pressure between the intakeand output. In the prior art, therefore, the two bushings are floatingin an axial sense.

[0005] Obtaining good leak-tightness between the intake and output isone of the principal objectives in the production of gear pumps. Infact, the efficiency of pumps of this type declines rapidly if theleak-tightness is not total. Another problem which the manufacturers ofpumps have to deal with is the noise of the pumps themselves, owing toirregular phenomena, or “ripples”, in the transfer of the fluid. A studyof the above-mentioned problems linked to the design of gear pumps isset out in “C. Bonacini, Sulla portata delle pompe ad ingranaggi (On theefficiency of gear pumps), L'ingegnere, 1961 n. 9”.

[0006] The above-mentioned solutions of the prior art have the commonproblem consisting in the noise of operation caused by the instantaneousoscillations of the output over time, better known as ripple noise. Theabove-mentioned oscillations generate a pulsating wave which, by way ofthe fluid, is transmitted to the surroundings and, in particular, to thewalls of the pump, to the pipes and to the output ducts. The noiseproduced can reach levels which are also unpredictable where theabove-mentioned members begin to resonate with the frequency ofoscillation or ripple.

[0007] In addition to this, the rotation of the gears causes a periodicvariation in the area of the inner face 17 b of the bushings 15 that isexposed to the output pressure. This variation determines oscillationsin the axial loads on the bushings, which contributes to an increase inthe noise of the pump, besides reducing the total efficiency thereof.This oscillation of the axial loads, which is normally of smallmagnitude in gear pumps having straight teeth, becomes significantlygreater in gear pumps having helical teeth, in which the meshing betweenthe gears is the cause of both mechanical and hydraulic axial loads suchthat the balance and the taking-up of clearances on the bushingsillustrated in FIGS. 1 and 2 is not completely satisfactory, since thehydraulic axial loads have perceptible pulses.

[0008] The object of the present invention is to provide apositive-displacement rotary pump which overcomes the disadvantages ofthe prior art and, in particular, which substantially reduces the noisewithout resulting in a substantial increase in the cost and complexityof production in comparison with pumps of known type. A further objectof the invention is to provide a pump which has good leak-tightnesscharacteristics between the intake and output, which is simple andeconomic to produce and maintain and which has good reliability overtime.

[0009] In order to achieve the above-mentioned objects, thesubject-matter of the invention is a positive-displacement pump whichcomprises the features indicated in the claims appended to the presentdescription.

[0010] One advantage of the present invention consists in that the axialposition of the rotors is unambiguously defined even in the event thatthey are subjected to axial loads or pressures owing to mechanicalcontact with the shell or portions thereof. In fact, it is known that,in the running-in stages of pumps of known type, it is accepted anddesirable for there to be contact of portions of the rotors with theshell so that the rotors remove an extremely small layer of materialuntil an individual seat has been “scooped out”, in such a manner that,when the pump is used after the running-in operation, the clearancebetween the teeth of the rotors and the shell has minimal dimensions.This slight interference between the helical teeth of the rotors and theshell of the pump produces additional axial loads on the rotors, whichmainly have an unknown value. The present invention, by providing afixed plane of reference, also ensures the correct positioning of therotors in the initial running-in stage of the pump, and even in theevent of interference between the rotors and the shell, when unknownaxial forces resulting from the above-mentioned mechanical contact areadded to the axial forces expected in normal operation of the pump.

[0011] Other characteristics and advantages of the invention will becomeclear from the following detailed description which is given withreference to the appended drawings which are provided purely by way ofnon-limiting example and in which:

[0012]FIGS. 1 and 2 illustrate, as described above, a gear pump of knowntype,

[0013]FIG. 3 is a longitudinal section of a gear pump according to thepresent invention, substantially similar to FIG. 1, in which identicalreference numerals identify corresponding elements,

[0014]FIG. 4 is a perspective view of a bushing set of the pump in FIG.3,

[0015]FIG. 5 is a plan view of an intermediate plate of the pump, takenin the plane of the line V-V in FIG. 3,

[0016]FIG. 6 is a plan view of the front cover of the pump, taken in theplane of the line VI-VI in FIG. 3,

[0017]FIG. 7 is a top view of a variant of one of the two components ofa bushing set, configured to promote the hydrodynamic lubricationthereof, and

[0018]FIG. 8 is a cross-section, drawn to an enlarged scale, taken online VII-VII in FIG. 7, in which the depth of the depressions orchannels of the bushing has been greatly increased for clarity ofdescription.

[0019] Now with reference to FIG. 3, a gear pump 20 comprises, asalready described above with reference to the known pump in FIGS. 1 and2, a shell 10 having an output opening and an intake opening for afluid, inside which are housed the driving gear 13 and the driven gear14. In the example in the Figure, the gears 13 and 14 are of thecylindrical type having helical teeth, but naturally the invention canalso be used with different gears, for example, a pair of straight spurgears, similar to that in the prior art illustrated in FIG. 1.

[0020] The shell 10 is closed at the two ends by the front cover 11 andthe rear cover 12. The end 21 of the shaft 23 of the driving gear 13protrudes from the front cover 11. For ease of illustration, the sealsbetween the shaft 23 and the front cover 11 have been omitted in FIG. 3.Inside the shell 10, the shafts 23 and 24 of the gears 13 and 14,respectively, are supported by two bushing sets, a front bushing set 15a and rear bushing set 15 b. Each of the bushing sets 15 a and 15 b canbe produced in one piece, as in the case of the known pump in FIG. 1, orpreferably in two separate pieces 22 a, 22 b, as illustrated in detailin FIG. 4. This latter preferred solution is more economical and precisefrom the point of view of production since it is easier to obtain veryhigh levels of working precision without any necessity for resorting tospecial machine tools. Furthermore, such a configuration of bushing setsminimizes the axial output passages for fluid, in the region of thecentral zones 27 of the bushing sets 15 a, 15 b which correspond to themeshing zone of the gears 13, 14. Longitudinal channels 25 arepreferably, but not in a limiting manner, provided on the two flanks ofeach bushing set 15 a, 15 b and promote the distribution of the outputpressure over the flanks of the bushing sets so as to keep the twoseparate pieces 22 a, 22 b close together.

[0021] The bushing sets 15 a, 15 b, both in the one-piece version and inthe version produced by means of separate pieces 22 a, 22 b, canpreferably have passages having variable width in order to allow thehydrodynamic lubrication thereof. One embodiment is illustrated in FIGS.7 and 8, wherein the face 17 b of each piece 22 a, 22 b of one or bothof the bushing sets 15 a, 15 b has depressions or channels 50 whichextend in a radial direction and whose profile-section is slightlyconcave in a direction orthogonal to the radius of the piece 22 a, 22 b.A slightly deeper slot or channel 51 is preferably provided in asubstantially central position in respect of each depression or channel50 for better distribution of the lubricating fluid. The behaviour inuse of a pump with bushing sets configured in this manner allows theperformance thereof to be further improved, especially during use athigh operating pressures.

[0022] At the end adjacent the rear cover 12, the shafts 23, 24 reactagainst a pair of check pins or balancing pistons 29, 30 which aremounted for axial sliding in a close-fitting manner in respective axialhousings 31, 32 which are provided in an intermediate plate 26. The endsof the check pins 29, 30 that are remote from the shafts 23, 24 aredirected towards a common chamber 34 which is provided in the rear cover12 and which, in use, is preferably in communication with the output ofthe rotary pump. In this manner, the pressurized fluid which will occupythe chamber 34 acts on the check pins 29, 30 so as to oppose the axialload produced by the gears 13, 14.

[0023] As is visible in FIG. 5, a groove 27 for accommodating a seal 16which is, for example, substantially configured in an “omega”-likemanner, is provided in the face of the intermediate plate 26. An opening28 is provided at the side of the plate that, when the pump is in use,communicates with the intake A. The configuration of the intermediateplate 26, which is similar to that of the lower cover of the pumps ofthe prior art in FIGS. 1 and 2, allows, during use of the pump, adistribution of output pressure M to be obtained over the outer face 17a of the rear bushing 15 b which substantially affects the area PMAXindicated with hatching in FIG. 5. This area, as is known, is greaterthan the area of the inner face 17 b of the rear bushing 15 b which issubjected to the output pressure, as long as the force differentialowing to the pressure provides for the production of an axial load onthe rear bushing 15 b directed towards the gears 13, 14, as indicated byarrow S′ in FIG. 3.

[0024] With reference now to FIG. 6, the inner face of the front cover11, unlike the pumps of known type, does not have the omega-like seal,and is instead provided with a large opening 36 which communicates withthe intake A of the pump. Two limbs 37 which communicate with theopenings 38, 39 for housing the shafts 23, 24 of the gears 13, 14 extendfrom the opening 36. This configuration of the face of the front cover11 ensures that the outer face 17 a of the front bushing 15 a issubjected only to the intake pressure, which affects, by way ofindication, the hatched area, which is denoted P_(MIN) in FIG. 6. Since,however, a portion of the inner face 17 b of the same front bushing 15 ais also subjected to the output pressure, the pressure differential actson the front bushing 15 a in a manner counter to that seen previouslyfor the rear bushing 15 b, as long as during use of the pump, the frontbushing 15 a, and in particular the two pieces 22 a, 22 b thereof, areurged firmly against the face of the front cover 11, as indicated byarrow S″ in FIG. 3.

[0025] At all times during use of the pump, the pressure applied to theshafts 23, 24 by the check pins 29, 30 is such that the gears 13, 14 arein turn thrust axially onto the front bushing set 15 a, as indicated byarrow S′″. Consequently, the axial forces S′, S″ and S′″ which areproduced during the operation of the pump all act in the same directionand contribute to keeping the gears 13, 14 and the front bushing set 15a and rear bushing set 15 b as a whole in abutment with the referenceplane, indicated by line VI-VI in FIG. 3, which is constituted by theface of the front cover 11 that is directed towards the inside of theshell 10.

[0026] In this manner, there is obtained a taking-up of the axialclearances which is complete and constantly defined in spite of theoscillation of the axial forces produced during the rotation and themeshing of the gears 13, 14.

[0027] As is visible in FIG. 3, the check pins 29, 30 have differentdiameters in order to apply different axial loads to the two gears 13,14. This is because, in the example of the Figure, the driving gear 13and driven gear 14 are of a helical type and therefore, duringoperation, produce per se axial loads whose direction is counter to andin accordance with the direction of the axial load applied by the checkpins 29, 30. Naturally, in the case of gears having straight teeth whichdo not produce per se axial loads, the check pins 29, 30 can havesubstantially corresponding diameters so as to apply an axial load ofequal intensity to both of the gears.

[0028] Naturally, the principle of the invention remaining the same, theforms of embodiment and details of construction may be varied widelywith respect to those described and illustrated, without therebydeparting from the scope of the present invention.

1. Positive-displacement rotary pump comprising a pair of meshing gears(13, 14) or rotors, consisting of a driving gear and a driven gear,which are contained in a shell (10) having an output opening and anintake opening for a fluid, the gears (13, 14) comprising shafts (23,24) which are supported by bushings (15 a, 15 b) having two faces (17 a,17 b) subjected, in use, to pressures which bring about an axial load(S′, S″) on the bushing itself, characterized in that the resultant ofthe axial loads (S′, S″) on the two bushings has a predetermineddirection so as to move the bushings (15 a, 15 b) and the gears (13, 14)as a whole into close abutment with a predetermined reference plane(VI-VI).
 2. Positive-displacement pump according to claim 1,characterized in that the axial loads (S′, S″) on the bushings (15 a, 15b) always act in the same direction as one another towards the referenceplane (VI-VI).
 3. Positive-displacement pump according to claim 1 or 2,characterized in that the gears (13, 14) are of the type having helicalteeth, the positive-displacement pump comprising a system forcompensating for the loads, including axial load means (29, 30) whichact on the same end of each shaft (23, 24) of the gears (13, 14), theaxial load means (29, 30) causing, in use, an axial load (S′″) on thegears (13, 14) which prevails over the axial forces produced by themeshing and having the same direction as the resultant of the axialloads (S′, S″) on the bushings (15 a, 15 b).
 4. Positive-displacementpump according to claim 3, characterized in that the axial load meanscomprise check pins or balancing pistons (29, 30) which are mounted foraxial sliding in a tight manner in respective housings (31, 32) whichare provided in a plate (26) fixed to one of the two covers (11, 12) ofthe shell (10) of the pump.
 5. Positive-displacement pump according toclaim 4, characterized in that the ends of the check pins (29, 30)press, at one side, on the ends of the shafts (23, 24) of the gears (13,14) and, at the other side, are directed towards a common chamber (34)which is provided in the cover (12) of the shell (10), the commonchamber (34) being in communication with the output opening of therotary pump.
 6. Positive-displacement pump according to claim 2,characterized in that the axial load means (29, 30) have differentdimensions so as to apply different axial loads (S′″) to each of thegears.
 7. Positive-displacement pump according to claims 3 and 6,characterized in that the check pins have different diameters. 8.Positive-displacement pump according to any one of the preceding claims,characterized in that at least one bushing (15 a, 15 b) comprises atleast two separate pieces (22 a, 22 b), each of which is intended tosupport in rotation an end of one of the two shafts (23, 24) of thegears (13, 14).
 9. Positive-displacement pump according to claim 8,characterized in that at least one longitudinal channel (25) is providedon the flanks of each separate piece (22 a, 22 b) of the at least onebushing (15 a, 15 b) and promotes the distribution of the outputpressure over the flanks of the bushing (15 a, 15 b), so as to keep thetwo separate pieces (22 a, 22 b) close together. 10.Positive-displacement pump according to claim 1, characterized in thatthe outer face (17 a) of one (15 a) of the two bushings is directedtowards the reference plane (VI-VI) and is affected only by the intakepressure (P_(MIN))11.
 11. Positive-displacement pump according to claim10, characterized in that a portion (P_(MAX)) of the outer face (17 a)of the other one (15 b) of the two bushings is affected by the outputpressure (M), the portion (P_(MAX)) being separated from the remainingportion of the outer face (17 a), which is affected by the intakepressure (A), by a separating seal (16) which is connected to a plate(26) directed towards the outer face (17 a) of the bushing (15 b), insuch a manner that the resultant of the pressures on the outer face (17a) of the bushing (15 b) is constantly greater than the resultant of thepressures acting on the inner face (17 b).
 12. Positive-displacementpump according to claim 1, characterized in that depressions orlocalized channels (50) are provided on one (17 b) of the two faces ofat least one of the bushings (15 a, 15 b) in order to allow thehydrodynamic lubrication thereof.
 13. Positive-displacement pumpaccording to claim 12, characterized in that the depressions or channelsextend radially over the face (17 b) of the bushing, the profile-sectionof each depression being slightly concave in the direction orthogonal tothe radius, a slightly deeper slot or channel (51) being provided in asubstantially central zone of each depression (50).